1. Field of the Invention
The present invention relates to an image processing apparatus, a control method therefor, and a storage medium. In particular, the present invention relates to an image processing apparatus for forming a toner image on a recording medium using a combination of a color toner and a transparent toner, a method for controlling the same, and a storage medium.
2. Description of the Related Art
In the field of laser printers and copiers that form an image by electrophotography, attention is focusing on not only traditional electrophotographic full-color printing using a color toner but also multicolor printing using a special toner, in addition to a color toner. The color toners indicate toners corresponding to four colors: cyan (C), magenta (M), yellow (Y), and black (Bk).
Examples of the special toner include a transparent toner that improves glossiness and protection of a toner image and a light toner that can suppress roughness of a highlighted part. The use of the special toner can add new value different from that in normal digital printing, and can further expand the range of digital printing.
Examples of the case where the transparent toner is used can be the case where the transparent toner is applied over the entire surface of an image with the aim of preventing toner separation and improving glossiness and the case where the transparent toner is partly applied as a watermark.
Applying a transparent toner can be classified into two methods: 1-pass and 2-pass (see, for example, Japanese Patent Laid-Open No. 2002-318482).
A 1-pass operation is described below using a multifunction peripheral (MFP) 101 illustrated in FIG. 2.
A photosensitive drum 217 is first charged and then exposed using a laser beam from a semiconductor laser 213. Color toners M, C, Y, and Bk and a transparent toner CL are attached to the photosensitive drum 217 sequentially using developing units 219 to 223. After the completion of attaching all of the toners to the photosensitive drum 217, the toners are transferred from the photosensitive drum 217 to a transfer drum 224. The transfer drum 224 is brought into intimate contact with a sheet of paper, electric charges are provided from the back of the sheet, and the toners are transferred to the sheet. At the last step, heat and pressure are applied to the sheet to which the toners adhere, and the toners are fixed to the sheet. The fixing condition for this case is the one in which the transparent toner and color toners are fixed at the same time.
A 2-pass operation is described next below using the same FIG. 2. The photosensitive drum 217 is first charged and then exposed using a laser beam from the semiconductor laser 213. The color toners M, C, Y, and Bk are attached to the photosensitive drum 217 sequentially using the developing units 219 to 222. After the completion of attaching all of the C, M, Y, and Bk color toners to the photosensitive drum 217, the toners are transferred from the photosensitive drum 217 to the transfer drum 224. The transfer drum 224 is brought into intimate contact with a sheet of paper, and electric charges are provided from the back of the sheet, and the toners are transferred to the sheet. Then, heat and pressure are applied to the sheet to which the toners adhere, and the toners are fixed to the sheet. This type of printing, which uses only a color toner, is called pre-printing.
The printed output (pre-printed sheet) with the C, M, Y, and Bk color toners being fixed is set on a paper feed tray 225 again. The photosensitive drum 217 is charged again and then exposed using a laser beam from the semiconductor laser 213. The transparent toner (CL) is attached to the photosensitive drum 217 using the transparent toner developing unit 223. After the completion of attaching the transparent toner to the photosensitive drum 217, the toner is transferred from the photosensitive drum 217 to the transfer drum 224. The transfer drum 224 is brought into intimate contact with a sheet of paper, electric charges are provided from the back of the sheet, and the toner is transferred to the sheet. At the last step, heat and pressure are applied to the sheet to which the toner adheres, and the toner is fixed to the sheet. The fixing condition for this case is the one in which the transparent toner and color toners are fixed in separate processes.
In the foregoing description, the same MFP 101 performs pre-printing and printing using the transparent toner (hereinafter referred to also as transparent-toner printing). However, different MFPs may carry out the 2-pass process. For example, an MFP 103 performs pre-printing, whereas the MFP 101 performs transparent-toner printing.
Concepts of 1-pass and 2-pass processes are illustrated in FIGS. 17A, 17B, and 17C. FIG. 17A illustrates a concept of the 1-pass process. FIG. 17B illustrates a concept of the 2-pass process using different MFPs (1701, 1702). FIG. 17C illustrates a concept of the 2-pass process using the same MFP.
The 2-pass process requires work of manually setting a pre-printed sheet on the paper feed tray of an MFP, but this process is advantageous in that the use of two image processing apparatuses capable of high-speed printing allows a printed output on which the transparent toner is applied to be obtained at high speed. In contrast, the 1-pass process enables a printed output on which the transparent toner is applied to be readily obtained because the 1-pass process does not require setting a sheet on the paper feed tray.
In the case of the 1-pass process, if the total amount of color toners applied to a sheet is large, the amount of a transparent toner adherable is limited. In contrast, in the case of the 2-pass process, the transparent toner can adhere to a sheet irrespectively of the total amount of color toners applied to the sheet, so desired glossiness is always achievable.
In consideration of these characteristics, one possible case is the one in which in a printing site that requires frequent proofreading the simple 1-pass process is used in the proofreading as a trial print run and in actual printing the 2-pass process capable of achieving high glossiness is used.
However, a problem arises in which there is a difference in values such as lightness, saturation, or hue for a printed output depending on the difference between toner characteristics or fixing conditions. For example, there is a difference between a signal value of a printed output on which a transparent toner is applied through the 1-pass process and that of a printed output on which a transparent toner is applied through the 2-pass process even if the same amount of the same transparent toner is applied to an image prior to fixation of the transparent toner. This is because the 1-pass process and 2-pass process have different numbers of fixing and pressures used in fixing.